Technical Field
The invention relates to a method of manufacturing a nitride semiconductor device.
Description of the Related Art
A known configuration of a semiconductor device includes a semiconductor layer that is made of a nitride semiconductor, for example, gallium nitride (GaN) (for example, JP 2014-086698A). Ion implantation may be employed to form a P-type region in a specified area of the semiconductor layer.
JP 2014-086698A describes a method that forms a silicon dioxide (SiO2) layer of 50 nm in thickness on a gallium nitride layer to serve as a protective layer for protecting the surface of the gallium nitride layer, performs ion implantation and performs heat treatment at temperature of 800° C. to 900° C. JP 2014-041917A describes a method that performs ion implantation into a gallium nitride layer without providing a protective layer, forms an aluminum nitride (AlN) layer to protect the surface of the gallium nitride layer after performing the ion implantation and performs heat treatment. The relevant techniques are described in WO 2015/029578 and JP 2009-126727A.
In the technique described in JP 2014-086698A, however, the protective layer is formed rather thin to allow for ion permeation and has insufficient thickness as the protective layer in heat treatment. There is accordingly a likelihood that nitrogen present in the gallium nitride layer is dropped off during heat treatment to roughen the surface of the gallium nitride layer. In the technique described in JP 2014-041917A, on the other hand, the protective layer may be denatured by heat during heat treatment. This may make it difficult to remove the protective layer. There is accordingly a need to expose the gallium nitride layer to rather extreme conditions for removal of the protective layer. This may result in roughening the surface of the gallium nitride layer.
There is accordingly a demand for a technique that suppresses the surface of the nitride semiconductor layer from being roughened by ion implantation or by heat treatment.